A good thank-you note is a clear and ruddy piece of prose. There are only two moves involved. First you remind the person what you are thanking them for. Then you tell them why. That's it. You sign off, sure. And you might throw in an extra sentence or two for a laugh or a private joke. But it's mostly a chop-chop exercise: two solid, sincere sentences, each touching on the heart of the matter. It isn't all that tough.